Next generation semiconductor devices and particularly those based on compound semiconductors such as gallium nitride have been proposed for high power and/or high frequency devices such as high electron mobility transistors (HEMTs). One limitation to enabling the full benefits of compound semiconductors such as GaN to be realized is that of thermal management. To this end, it has been proposed to integrate a synthetic diamond heat spreading substrate, which has very high thermal conductivity, in close proximity with a semiconductor device structure, such as a GaN based semiconductor device structure, to enable higher power density usage, higher device packing densities, and/or to increase the lifetime of such devices.
In recent years, methods of fabricating high quality single crystal GaN on single crystal silicon substrates have been developed. These methods typically involve the provision of strain management layers immediately over the silicon substrate on which the single crystal GaN is epitaxially grown to alleviate strain resulting from lattice mismatch between the single crystal silicon substrate and the single crystal GaN material. As such, high quality single crystal GaN on silicon substrates is now commercially available. However, one problem with such substrates is that the thermal conductivity of the silicon material is relatively low and thus the overlying GaN cannot be driven to its full potential without thermal management issues arising.
In addition to the above, methods of fabricating high quality single crystal GaN on single crystal silicon carbide substrates have also been developed. Silicon carbide has a higher thermal conductivity than silicon. Again, the methods typically involve the provision of strain management layers immediately over the silicon carbide substrate on which the single crystal GaN is epitaxially grown to alleviate strain resulting from lattice mismatch between the single crystal silicon carbide substrate and the single crystal GaN material. As such, high quality single crystal GaN on silicon carbide substrates is now commercially available and for certain applications such substrate wafers are preferred due to certain characteristics of the GaN, e.g. low defect density, in addition to the improved thermal performance of the silicon carbide based substrate. One drawback is that these silicon carbide based substrates are more expensive than the silicon based alternative. Furthermore, although the thermal conductivity of the silicon carbide is higher than silicon, the overlying GaN still cannot be driven to its full potential without thermal management issues arising.
One possible alternative to the silicon and silicon carbide solutions which are currently available is to develop a method of fabricating high quality single crystal semiconductor layers such as GaN on a synthetic diamond substrate which has much higher thermal conductivity than other thermal management materials. In this regard, it is possible to grow semiconductors such as GaN on a diamond substrate, usually with a thin single crystal silicon or silicon carbide layer disposed on the diamond substrate with GaN being epitaxially grown on the thin silicon or silicon carbide layer. However, to date it has not been possibly to achieve the same quality of single crystal GaN material on such substrates due to strain management issues when compared with GaN growth on silicon or silicon carbide substrates. Furthermore, it is difficult to provide a high quality, low defect thin layer of single crystal silicon or silicon carbide on a diamond substrate which provides a good epitaxial substrate for semiconductor growth and which is also exceedingly thin (e.g. less than 50 nm thickness) to alleviate thermal barrier resistance problems between the overlying semiconductor and the underlying diamond heat spreading material.
In light of the above, an alternative approach has been proposed in which the GaN is grown on a silicon or silicon carbide substrate and then the GaN is transferred to a synthetic diamond substrate. Since single crystal semiconductors usually have an epitaxial layered structure (a so-called “epilayer” structure) which is optimized for a semiconductor device fabricated on an exposed upper surface, then in most instances it is desirable for the underlying silicon or silicon carbide substrate to be replaced by a synthetic diamond substrate rather than merely providing a diamond layer on the exposed upper surface of the GaN epilayer structure. That is, the transfer process involves the removal of the native growth substrate and the provision of a synthetic diamond substrate in its place. In such a process, two factors are of importance: (i) that the transfer process does not unduly damage the GaN epilayer structure; and (ii) that the synthetic diamond substrate is integrated in close thermal contact with the GaN epilayer structure with a low thermal barrier resistance between the active GaN epilayer structure and the synthetic diamond material.
U.S. Pat. No. 7,595,507 discloses a fabrication route to transfer a GaN epilayer structure from a native silicon substrate to a synthetic diamond based substrate. The methodology comprises: (i) providing a native growth substrate (e.g. silicon) on which a compound semiconductor (e.g. GaN) is disposed; (ii) bonding a carrier substrate to the compound semiconductor layer; (iii) removing the native growth substrate; (iv) forming a nucleation layer over the compound semiconductor layer; (v) growing polycrystalline CVD diamond on the nucleation layer, and then (vi) removing the carrier substrate to achieve a layered structure comprising the compound semiconductor bonded to the polycrystalline CVD diamond via the nucleation layer.
In the above-described fabrication route, usually the material selected for the intermediate carrier substrate will be the same as that of the native growth substrate to avoid introducing different stress management issues. If silicon is used for the native substrate and intermediate carrier substrate then these substrates can in principle be removed by, for example, conventional mechanical grinding techniques and/or a combination of grinding and etching. However, in this regard it has been found that while conventional grinding works for the growth substrate because the wafer is flat at this stage of the process, the removal of the intermediate carrier substrate after diamond growth is more problematic. This is because the intermediate carrier substrate is bowed after diamond growth and cannot easily be removed by conventional grinding.
U.S. Pat. No. 7,595,507 indicates that typical substrates used for growth of wide-gap semiconductors are sapphire, silicon carbide and silicon and that if the substrate is sapphire or silicon carbide whose removal is difficult, the substrate may be removed by chemical lift-off or laser lift-off. However, attempts to remove a silicon carbide intermediate carrier substrate after diamond growth have proved problematic. The silicon carbide cannot be easily etched either by wet or dry chemical processes. Furthermore, it cannot be easily removed by lapping because the bow of the silicon carbide-semiconductor-diamond composite wafer after diamond growth exceeds its thickness. Further still, attempts to remove the silicon carbide carrier substrate using a laser lift off technique have caused the diamond-semiconductor wafer to shatter when such a technique is utilized.
In light of the above, it is an aim of embodiments of the present invention to solve the aforementioned problem and provide a viable fabrication route for manufacturing diamond-semiconductor composite substrates starting with silicon carbide-semiconductor native wafers.